The present invention relates to the use of structured solid catalyst beds or monolithic catalysts for the efficient processing of gas/liquid feed streams, and more particularly to the processing of such feed streams to carry out hydrotreating and hydrogenation reactions at rapid rates and at high conversion efficiencies.
Fixed bed reactors packed with catalyst pellets in various forms, such as, beads, cylinders, wagon wheels, etc., have been widely used in the chemical processing and refining industries for hydrotreating and hydrogenation processes. Many of these processes are carried out as three-phase (gas-liquid-solid) processes wherein a gas/liquid feed stream is reacted over a solid catalyst. Hydrotreating is an important refinery process for the production of clean (low-sulfur) fuel from petroleum feedstocks, while hydrogenation is widely used for production of a variety of chemicals.
Commercial pellet bed reactors for carrying out such processes, commonly termed xe2x80x9ctrickle bedxe2x80x9d reactors, typically operate in a gas/liquid co-current downflow mode, i.e., a mode wherein both the gas and the liquid reactants flow in the same direction (downwardly) through the catalyst bed. Superficial liquid linear velocities in such reactors, calculated from the length of the catalyst bed and the average transit time for liquid through the bed, are in the range of about 0.01 to 2 cm/s. Trickle bed reactors have been successfully used in the industry for nearly half a century and represent a mature chemical processing technology. Refinements of this technology have involved optimizing catalyst size, shape, and packing method, and by tuning the operating regime, e.g., by employing somewhat higher liquid flow velocities. However, such improvements have been incremental in nature and have not resulted in major enhancements in the efficiencies of these processes.
Over the past two decades, a considerable amount of research effort has been devoted to the development of structured catalysts such as monolith or honeycomb catalysts. Honeycomb monoliths are widely used to support catalysts for the gas-phase processing of combustion engine exhaust gases in automotive catalytic converters. On the other hand, the use of monoliths for carrying out three-phase reactions involving the processing of gas/liquid feedstreams has been quite limited, these reaction systems being very different from gas phase reaction systems.
Of course the low-pressure-drop advantage of honeycomb monolith catalysts is well recognized, and various studies of the behavior of such catalysts have been reported in the literature. However, low pressure drop is only one characteristic affecting catalyst performance in these structures and much attention has focused on trying to understand other reaction performance factors operating in monolithic catalyst beds supporting gas/liquid catalytic reactions. Certainly, from a reactor engineering point of view, three-phase catalytic reaction processes are far more complicated than gas-phase catalytic reactions such as occur, for example, in the monolithic catalysts used in automotive catalytic converters.
In xe2x80x9cCatalytic Wet Oxidation Of Acetic Acid Using Platinum On Alumina Monolith Catalystxe2x80x9d, Catalysis Today, 40 (1998) 59-71, Klinghoffer et al. tested the oxidation of acetic acid over Pt/alumina monolith catalysts in an air/water system. Irandoust and Gahney studied the competitive hydrodesulfurization and hydrogenation of thiophene/cyclohexene over CoMo/alumina monolith catalysts with a simulated mixture in xe2x80x9cCompetitive Hydrodesulfurization And Hydrogenation In A Monolithic Reactorxe2x80x9d, AIChE Journal, Vol. 36, No.5, 746-752 (1990)).
Hatziantonlou et al. compared the hydrogenation of nitro-compounds over Pd/monolith catalysts to the same hydrogenation in a slurry reactor, but reported a lower reaction rate for the monolith reactor on a catalyst weight-for-weight basis (see xe2x80x9cMass Transfer And Selectivity In Liquid-Phase Hydrogenation Of Nitro-Compounds In A Monolithic Catalyst Reactor With Segmented Gas-Liquid Flowxe2x80x9d Ind. Eng. Chem. Process Des. Dev., 25 (1986) 964-970). In xe2x80x9cSelective Three-Phase Hydrogenation Of Unsaturated Hydrocarbons In A Monolithic Reactorxe2x80x9d Chemical Engineering Science, Vol. 51, No. 11, 3019-3025 (1996), Smits et al. tested olefin hydrogenation reactions over Pd/monolith catalysts, and found the reaction rate constants to be highly dependent on liquid linear velocity through the catalyst.
What each of these studies have failed to address is the question of whether such structured catalyst beds have any conversion efficiency advantages over conventional catalyst beds when used in a manner consistent with current commercial catalyst usage. Thus a substantial question remains whether honeycomb monolith or other structured catalyst beds can in fact be useful replacements for the commercial catalyst beds presently used in trickle bed or other conventional gas/liquid reactors. This question of practical utility remains largely unanswered because much of the conversion data reported in the literature has been generated on a laboratory scale in small xe2x80x9cdifferentialxe2x80x9d reactors, these providing low rates of one-pass conversion, in many cases below 50%. For economic reasons, large commercial reactors are often operated as xe2x80x9cintegralxe2x80x9d reactors, i.e., reactors designed to provide one-pass conversion rates well above 50%, and in many cases near 100%. Unfortunately, it is not possible to extrapolate laboratory findings based on differential reactor data to integral reactor performance, because of the complexity of the reaction coupling and mass transfer interactions involved.
Another uncertainty relates to the fact that previously recorded laboratory results are often predicated on the operation of monolith reactors in the so-called Taylor flow regime. Taylor flow refers to a flow mode characterized by the movement of alternating liquid slugs and gas bubbles of approximately equal size through the channels of a honeycomb catalyst. Maintenance of Taylor flow typically requires reactor operation at relatively high liquid linear velocities (e.g., 30 cm/s) and relatively low gas/liquid ratios (e.g., 0.5 VV). Controlling feed stream flows to meet these requirements may not be feasible in plants designed for carrying out hydrotreating and hydrogenation processes on commercial scales.
Thus it remains to be determined whether structured catalysts such as honeycomb monoliths can provide any advantages over conventional catalyst beds in commercial reactors and, if so, for what reactions and under what reaction conditions can such advantages be secured.
The present invention provides catalytic conversion processes useful for reactors containing xe2x80x9cmini-structuredxe2x80x9d catalyst beds wherein three-phase chemical reactions such as hydrogenation and hydrotreating can be carried out with high efficiency at conversion rates of practical utility for one-pass or integral reactor systems. By xe2x80x9cmini-structuredxe2x80x9d catalyst beds is meant catalyst beds that are divided into a number of small catalyzed reaction channels of hydraulic diameter in the order of 0.1 to 10 mm, these being exemplified by honeycomb catalysts of appropriate channel size. In catalyst beds of this structure, the bulk gas and liquid reactant streams are streamlined or divided into separate, small reactant streams that each undergo catalytic reactions inside separate channels.
The reactors used in these processes generally employ solid catalysts and the gas/liquid process feed streams are streams wherein the gas:liquid (G:L) ratio is high. That is, the processes are carried out using relatively high gas flows and relatively low liquid linear velocities to convey the gas/liquid feed streams through the honeycomb catalysts.
More particularly, the invention includes an improved process for carrying out a gas-liquid reaction in the presence of a solid catalyst. In accordance with that process a gas-liquid feed stream comprising gas and liquid reactants is first conveyed through a monolithic structured catalyst bed. The catalyst is provided as a honeycomb or assembly of honeycombs of conventionally channeled structure, i.e., comprising a honeycomb body incorporating a plurality of parallel, open-ended channels running from a first or entrance face to a second or exit face of the body, through which channels the fluids to be treated may freely pass. The channels are formed or bounded by channel walls, these preferably being provided in the form of thin, criss-crossing or intersecting webs of solid material wherein or whereupon the solid catalyst for treating the fluid stream being treated is provided.
In carrying out the process of the invention, the liquid making up the gas-liquid feed stream is conveyed through the honeycomb channels at controlled, limited flow rates. Effective treatment requires that the liquid flow rate be adjusted to insure that the superficial linear velocity of the liquid through the catalyst will be in the range of about 0.01-10 cm/s. The superficial liquid linear velocity of the liquid through the honeycomb catalyst bed is defined in accordance with convention to be the quotient of the length of the catalyst bed divided by the average time required for a small segment of the liquid feed to traverse the bed.
While the liquid is traversing the honeycomb catalyst bed the gas component of the feed stream is conveyed through the bed in a quantity that will maintain a high gas:liquid (G:L) volume ratio in the channels of the honeycomb. Generally, the gas will be provided in a proportion sufficient to maintain a gas:liquid ratio that is in the range of about 1-4000 standard (Normal) liters of gas (NL) for each liter (L) of liquid present, i.e., a G:L ratio from 1:1 to 4000:1 NL/L. For the purpose of the present description a standard or Normal liter of gas is a volume of gas that would occupy one liter at the standard temperature and pressure of 20xc2x0 C. and 1 atmosphere.
The processing conditions described are useful for carrying out a variety of three-phase reactions over monolithic honeycomb catalyst beds, but are particularly well suited for conducting hydrogenation or hydrotreating reactions using hydrocarbon feedstocks. A variety of unsaturated or substituted hydrocarbon compounds can be efficiently processed by this means. Treatable unsaturated hydrocarbons include the straight- or branched-chain alkenes or alkynes, as well as aromatic hydrocarbons and hydrocarbons incorporating unsaturated hydrocarbon functional groups. Substituted hydrocarbons include those comprising sulfur- or nitrogen-containing functional groups.
It is a particular advantage of the process of the invention that difficult measures to maintain Taylor flow conditions in the honeycomb channels are not required. Instead, one-pass conversion rates in excess of 50% and more typically in excess of 80% of the theoretical conversion limit, are readily obtained for these hydrocarbon reactants regardless of whether Taylor flow is maintained or not, provided the process is conducted within the herein-prescribed ranges of liquid flow and G:L ratio. Thus these processes are suitable for use in single-pass or integrated reactor systems without the need for high recycle rates or supplemental processing equipment.